Effect of hydrogen on the superplasticity of Ti600 alloy

The influence of hydrogen on microstructure evolution and superplastic behavior of a new near α high-temperature titanium alloy-Ti600 alloy were studied. The results show that hydrogen increases the amount of β phase and δ hydride with fcc structure exists in the specimens when the hydrogen content is over 0.3 wt.%. After hydrogenation, the deformation temperature of Ti600 alloy can be decreased about 80 °C and the strain rate can be increased by at least one order. Addition of proper hydrogen can reduce the flow stress of Ti600 alloy significantly. The flow stress of Ti600–0.5H alloy decreases about 78% of that unhydrogenated Ti600 alloy at 840 °C and 5 × 10⁻⁴ s⁻¹. Moreover, introducing hydrogen into Ti600 alloy decreases the dislocation density, promotes the dislocation motion and facilitates the β phase flow.     

Hydrogenation study of suction-cast Ti40Zr40Ni20 quasicrystal

Suction casting was predicted to be an usable method for improving the hydriding kinetics of Ti/Zr-based icosahedral quasicrystals (IQCs) in our previous work. To further determine it, a suction-cast Ti₄₀Zr₄₀Ni₂₀ IQC alloy was used for hydrogenation studies by Pressure Composition Isotherm (PCI) and Temperature Programmed Desorption (TPD) techniques. The results showed that, this alloy absorbed hydrogen rapidly with obvious hydrogen pressure plateau and some reversibility, however, displayed very limited hydrogen capacity (about 0.7 wt.%) and low equilibrium pressure. After several hydrogenation/dehydrogenation cycles, the IQC structure transformed into two hydride phases, ZrH_2−x and one unknown, both of which decomposed at above 600 °C, suggesting high thermo-stability for them. On the whole, indeed the suction-casting method can increase the hydrogen absorption rate of Ti/Zr-based IQCs, however, the hydrogenation properties of the Ti₄₀Zr₄₀Ni₂₀ IQC alloy still need a mighty advancement.     

Effect of hydrogen on the microstructure and mechanical properties of high temperature deformation of Ti6Al4V additive manufactured

The additive manufactured Ti6Al4V-xH titanium alloy was compressed at 600°C–750 °C on a Gleeble 3800 testing machine, and the compression rates were 1s⁻¹ and 0.01s⁻¹, respectively. The experimental results show that with the increase of hydrogen content, the flow stress of the alloy decreases firstly and then increases gradually. When the hydrogen content is 0.27 wt%, the flow stress of titanium alloy is the smallest. EBSD and TEM analysis were carried out and show that the α lamellar microstructure became larger at 0.27H, the corresponding flow stress also decreased, and slip bands appeared in the alloy. Dislocation slip was an important deformation mechanism of the alloy. When the hydrogen content continued to increase, the α phase in the alloy gradually decreased, and α″ appeared at 0.81H. Therefore, adding appropriate hydrogen can reduce the alloy flow stress and improve the performance of titanium alloy during hot deformation.     

Effect of Cr addition on room temperature hydrogenation of TiFe alloys

This paper discusses the effect of AB₂ (Ti(Cr, Fe)₂) phase on the hydrogenation properties of a Ti–Fe–Cr alloy system. Five Ti–Fe–Cr based alloys were fabricated by varying the Cr content. The microstructural analysis results revealed that the fraction of the Ti(Cr, Fe)₂ phase increased with the increasing Cr content. The first hydrogenation test results indicated that all the alloys could absorb a significant amount of hydrogen at room temperature (30 °C) without a separate activation process. This behavior improved when the Ti(Cr, Fe)₂ phase existed in the AB phase; the kinetics of the first hydrogenation tended to increase with the fraction of Ti(Cr, Fe)₂ phase. The enhancement in the first hydrogenation kinetics of the Ti–Fe–Cr based alloys was attributed to the synergetic effect of the interface between the AB and Ti(Cr, Fe)₂ phases and the inherent fast hydrogenation of the Ti(Cr, Fe)₂ phase. However, the total hydrogen storage capacity decreased when the fraction of Ti(Cr, Fe)₂ phase increased.     

Characteristic and kinetics of hydrogen absorption during the heat preservation stage and the cooling stage of TC21 alloy

TC21 alloy is hydrogenated under different initial hydrogen pressures at hydrogenation temperatures in the range of 450 °C–850 °C. Hydrogen absorption characteristic and kinetics during the heat preservation stage and cooling stage, hydrogen content and activation energy are investigated. The hydrogen absorption reaches equilibrium first at higher hydrogenation temperature and initial hydrogen pressure during the heat preservation stage. The hydrogen absorption reaches equilibrium first at lower hydrogenation temperature and initial hydrogen pressure during the cooling stage. Mechanisms of hydrogen absorption are analyzed during the heat preservation stage and the cooling stage. Phase compositions of the hydrogenated TC21 alloys are analyzed by XRD. Hydrogen content increases first and then decreases, then increases slightly, and finally decreases with the increase of hydrogenation temperature. Hydrogen content increases gradually with the increase of initial hydrogen pressure. The activation energy of hydrogen absorption in TC21 alloy is about 18.304 kJ/mol.     

Effects of hydrogen on the interfacial reaction between Ti6Al4V alloy melt and Al2O3 ceramic shell

Effects of hydrogen on the interfacial reaction between Ti6Al4V (Ti64) alloy melt and Al₂O₃ ceramic shell were studied by arc-melting in H₂/Ar gaseous mixture. The thickness of interfacial reaction layer between Ti64 alloy melt and Al₂O₃ ceramic shell without hydrogenation is 80 μm. But the thickness of reaction layer can be reduced to 10 μm at a hydrogen partial pressure of 20 kPa, which is better than the thickness of interface layer between Ti64 alloy and widely used Y₂O₃ shells. At Ti64 casting surface, the content of β-soft ductile phase is increased and the width of both reaction zone and transition zone are reduced with the increase of hydrogen partial pressure, thus reduces the hardness and the element diffusion distance. Therefore, Hydrogen can effectively control the interfacial reaction in the process of titanium alloy investment casting, and Al₂O₃ ceramic will be a kind of very promising shell material.     

Effect of introducing manganese as additive on microstructure,hydrogen storage properties and rate limiting step of Ti–Cr alloy

TiCr₂ with adding different amount of Mn (0, 2, 4 and 8 wt.%) alloys have been investigated. All alloys have C14-type main phase (gray color in SEM) and Ti minor phase (dark gray color in SEM). Rietveld fitting results proved that the lattice parameter a and cell volume of C14-type phase decreased with increasing Mn content. The first hydrogenation measurement manifest that all alloys have best activation properties and could be activated without any prior heat treatment and hydrogen exposure. However, introducing Mn led to the decrease of the first hydrogen absorption rate of TiCr₂ alloy, which may be due to the decrease of cell volume of C14-type main phase. The first hydrogenation properties at low temperature and effect of air exposure of the alloy were discussed. The results showed that the maximum hydrogen absorption capacity at 0 °C was obviously higher than that at room temperature. In addition, TiCr₂ alloy doped with minor amounts of Mn after long-time air exposure showed better hydrogenation performance. This may be due to the Mn additive acting as a deoxidizer. Finally, the first hydrogenation kinetic mechanisms of all alloys at different temperature were also studied by using the rate limiting step.     

Gradient microstructure of TC21 alloy induced by hydrogen during hydrogenation

Through melt hydrogenation, a gradient microstructure (α″ + α′)/(α + β_H) has been observed in TC21 alloy. The addition of hydrogen induces martensite transformation and increases the volume fraction of β. It is found that the absorption process of hydrogen atoms can be divided into melting and cooling stages. During cooling, the continuous absorption of hydrogen and the corresponding decrease of freezing point of melt extend solidification time of melt and lead to hydrogen enrichment in the upper of the specimen, which induces the formation of the gradient structure. The hydrogenated TC21 alloy shows higher thermoplasticity compared with the unhydrogenated TC21 alloy. The flow stress of the upper part of the hydrogenated alloy is lower than that of the center part. A gradual variation has been observed in the microhardness along the gradient direction due to variation in the microstructure. The microhardness of the upper surface drops about 45% with 14.6 at.%H.     

Strengthening versus softening mechanisms by hydrogen addition in β-Ti40 alloy

The effects of hydrogenation (≈0–0.9 wt.%) on the flow stress behavior and microstructural evolution of Ti40 (Ti–25V–15Cr–0.2Si) alloys during hot deformation are investigated. Isothermal hot compression tests are performed in the temperature range of 1023–1223 K with a strain rate of 0.01 s⁻¹. The stress–strain curves of the Ti40-xH alloys are recorded and the relationship between steady-state stress and hydrogen content at different temperatures is determined. With an increase of hydrogen content, the steady-state flow stress initially increases, and then decreases before increasing again at higher hydrogen contents. This behavior implies that hydrogen exerts a strengthening effect at low and high concentrations and a softening effect at intermediate concentrations. The strengthening mechanism is explained in terms of solid solution strengthening of hydrogen, fine grain strengthening caused by hydrogen induced DRX (dynamic recrystallization) and precipitation strengthening of silicide. The softening mechanism is explained in terms of DRX, grain strength softening caused by grain growth and a continuous reticular precipitation of silicide.     

Hydrogen absorption behavior of non-stoichiometric Zr7-xTixV5Fe (x = 0, 0.3, 0.9, 1.5 and 2.1) alloys

Herein, the influence of Ti substitution on microstructure and hydrogen absorption behavior of annealed Zr_xTi_7-xV₅Fe (x = 0, 0.3, 0.9, 1.5 and 2.1) alloys is systematically studied. The results reveal that the Ti-substituted alloys contain only α-Zr and C15–ZrV₂ phases, where the content of C15–ZrV₂ phase initially increased with increasing Ti content, followed by a gradual decrease. On the other hand, the content of α-Zr phase decreased with increasing Ti content, and then increased. Hence, the cell volume of C15–ZrV₂ phase is maximum and the cell volume of α-Zr phase is minimum at x = 1.5, corresponding to Zr_5.5Ti_1.5V₅Fe alloy. Moreover, the results exhibit that the plateau pressure of α-Zr phase increased with increasing Ti content at 623 K, 673 K and 723 K, whereas the plateau pressure of C15–ZrV₂ phase exhibited the reverse change. Also, the stoichiometric ratio (A/B) of A-side element to B-side element in α-Zr phase gradually decreased with increasing Ti content, whereas the C15–ZrV₂ phase exhibited an opposite trend. One should note that the A/B stoichiometric ratio may play a critical role in determining the plateau pressure of both phases. The hydrogen absorption curves of Zr_xTi_7-xV₅Fe alloys showed that the hydrogen absorption content increased with increasing Ti content. It should be noted that the hydrogen absorption kinetics decreased with increasing Ti content, which may be mainly caused by increasing of the particle size with increasing Ti content.     

Self-ignition combustion synthesis of oxygen-doped TiFe

This paper describes the Self-Ignition Combustion Synthesis (SICS) of the hydrogen storage alloy Ti_1.15FeO_x (x = 0, 0.024, and 0.050) in a hydrogen atmosphere and the obtained product's hydrogenation properties. Ti, Fe, and α-Fe₂O₃ powders were mixed to produce Ti_1.15FeO_x and uniformly heated to the eutectic temperature 1358 K for self-ignition, which occurred after the hydrogenation and decomposition of Ti. The X-ray diffraction results for the final product had TiFeH_0.06 and Fe₂Ti as the major and minor phases, respectively. The hydrogen storage capacity of the product at a hydrogen pressure of 5 MPa was 1.18–1.50 m%. As the value of x increased, the capacity gradually decreased. However, the initial hydrogenation kinetics of the product improved. In contrast, the equilibrium pressure for the product was almost unaffected. These results demonstrate that SICS is quite effective for producing O-doped TiFe alloys as well as TiFe without rare metal. The procedure offers many benefits of releasing tough activation treatment, minimizing operation time, and saving energy.     

Facilitating de/hydrogenation by long-period stacking ordered structure in Mg based alloys

We propose a simple strategy to effectively improve the hydrogenation and dehydrogenation kinetics of Mg based hydrogen storage alloys. We designed and prepared an Mg_91.9Ni_4.3Y_3.8 alloy consisting of a large quantity of long-period stacking ordered (LPSO) phases. A type of highly dispersed multiphase nanostructure, which can markedly promote the de/hydrogenation kinetics, has been obtained utilizing the decomposition of LPSO phases at first a few of hydrogenation reactions. The fine structures of LPSO phases and the microstructural evolutions of the alloy during hydrogenation and dehydrogenation reactions were in detail characterized by means of transmission electron microscopy (TEM). The LPSO phases transformed to MgH₂, Mg₂NiH₄, and YH₃ after the first hydrogenation. The highly dispersed nanostructure at macro and micro (nano) scale range remains even after several de/hydrogenation cycles. The alloy shows excellent hydrogen storage properties and its reversible hydrogen absorption/desorption capacities are about 5.8 wt% at 300 °C. Particularly, the alloy exhibits very fast dehydrogenation kinetics. The dehydrogenated sample can release approximately 5 wt% hydrogen at 300 °C within 200 s and 5.5 wt% within 600 s. We elucidate the structural mechanism of the alloy with outstanding hydrogen storage performance.     

Effect of thermo hydrogen treatment on lattice defects and microstructure refinement of Ti6Al4V alloy

An investigation of a rolled Ti6Al4V alloy after thermo hydrogen treatment was performed. The effect of hydrogen content on the types and amount of lattice defects, the microstructure refinement after hydrogenation–dehydrogenation processing and the refining mechanisms were studied. The results show that the types of defects are at first vacancies and dislocations, and then they are mainly dislocations with increasing of hydrogen content. The amount of defects increases gradually with increasing of hydrogen content. After thermo hydrogen treatment, the rolled microstructure of Ti6Al4V alloy is refined.     

Optimizing Ti–Zr–Cr–Mn–Ni–V alloys for hybrid hydrogen storage tank of fuel cell bicycle

AB₂-type Ti-based alloys with Laves phase have advantages over other kinds of hydrogen storage intermetallics in terms of hydrogen sorption kinetics, capacity, and reversibility. In this work, Ti–Zr–Cr-based alloys with progressive Mn, Ni, and V substitutions are developed for reversible hydrogen storage under ambient conditions (1–40 atm, 273–333 K). The optimized alloy (Ti_0.8Zr_0.2)_1.1Mn_1.2Cr_0.55Ni_0.2V_0.05 delivers a hydrogen storage capacity of 1.82 wt%, the hydrogenation pressure of 10.88 atm, and hydrogen dissociation pressure of 4.31 atm at 298 K. In addition, fast hydrogen sorption kinetics and low hydriding-dehydriding plateau slope render this alloy suitable for use in hybrid hydrogen tank of fuel cell bicycles.     

Mg3Cd: A model alloy for studying the destabilization of magnesium hydride

We prepared an ordered Mg₃Cd alloy by high energy ball milling of elemental powders. The synthesized alloy exhibited good hydrogenation kinetics and reversibly absorbed about 2.8 wt. % of hydrogen. The temperature dependence of hydrogenation kinetics of the alloy measured in the range of temperatures covering the order-disorder phase transformations in the Mg₃Cd and MgCd phases did not exhibit any anomalies and could be fitted with a single Arrhenius line. The measured apparent activation energy (69 ± 2 kJ/mol) hinted that hydrogenation process was controlled by diffusion of Cd in metallic phase. The pressure-composition isotherms exhibited negligible pressure hysteresis and sloping pressure plateau. Based on microstructural evidence obtained with the aid of X-ray diffraction and scanning electron microscopy, we built a thermodynamic model predicting the plateau hydrogen pressure for partially hydrogenated alloy. The predictions of the model were in a good agreement with the experimental data. Finally, we discussed the origins and the growth mechanisms of Cd whiskers observed in the alloys after full hydrogenation cycle.     

Hydrogen solubility in molten TiAl alloys

In this work, the hydrogen solubility in a titanium–aluminium (TiAl) binary alloy melt was investigated through a theoretical analysis and the results compared subsequently with values determined experimentally. Determination of the theoretical values of hydrogen solubility is based on a modified version of Sievert’s law, in which hydrogen solubility is related to the activity coefficient of the alloy melt and the hydrogen solubility in pure liquid metals. The activity coefficient is obtained in terms of the free volume theory, in which excess entropy is sufficiently taken into account. The experimental values of the hydrogen solubility in the two alloy melts, Ti45Al and Ti47Al, were determined to validate the calculated values. This was performed using hydrogen charging apparatus. The experimental values obtained were in good agreement with the calculated values.     

Hydrogen storage properties and microstructure of melt-spun Mg90Ni8RE2 (RE = Y, Nd, Gd)

For hydrogen storage applications a nanocrystalline Mg₉₀Ni₈RE₂ alloy (RE = Y, Nd, Gd) was produced by melt spinning. The microstructure in the as-cast, melt-spun and hydrogenated state was characterized by X-ray diffraction and electron microscopy. Its activation, hydrogenation/dehydrogenation properties and cycle stability were examined by thermogravimetry in the temperature range from 50 °C to 385 °C and pressures up to 30 bar H₂. It was found that the activated alloy can reach a reversible gravimetric hydrogen storage density of up to 5.6 wt.%-H. Furthermore, the reversible gravimetric hydrogen storage density increases with the number of hydrogenation/dehydrogenation cycles, while the dehydrogenation rate remained unchanged. This observation was attributed to the increase of the specific surface area of the ribbon due to cracking during repeated cycling. However, the microstructure of the hydrogenated alloy remained nanocrystalline throughout cycling.     

Understanding first cycle hydrogenation properties of Ti–Fe–Zr ternary alloys

Zirconium has been used as an alloying element to mitigate the difficulty in the initial hydrogenation of pristine TiFe, a room-temperature hydrogen storage material. The addition of zirconium induces the formation of AB₂-type ternary phases such as τ1 and τ3. In this study, these ternary phases were produced individually and their activation kinetics and hydrogen storage properties were assessed. Ultimately, both τ1 and τ3 can be activated at room temperature under hydrogen at a pressure of 3 MPa. The result validates the presumption that Zr-added TiFe is easily activated with the help of the Ti–Fe–Zr ternary phases. In addition, they can store 1.8–1.9 wt% of hydrogen, which proves their own use as a hydrogen storage material. The easy activation may be caused by the compositional inhomogeneity of mixed metal oxide at the alloy surface, as revealed by depth-profiling X-ray photoelectron spectroscopy; this provides new reaction pathways for hydrogenation.     

Remarkable enhancement in durability of ZrCo alloy against hydrogen induced disproportionation by Ti and Nb co-substitution

Considering the thermodynamic stability of various hydrides, a strategy has been employed to improve the hydrogen isotope storage properties of ZrCo alloy which involves partial co-substitution of Zr with Ti and Nb. Herein, alloys of composition Zr_0.8Ti_0.2-xNb_xCo (x = 0.05, 0.1, 0.15) is prepared, characterized and the effect of Ti and Nb doping on hydrogen storage properties of parent ZrCo alloy is investigated. XRD analysis confirmed the formation of desired pure cubic phase of all the synthesized alloys similar to ZrCo phase. The presence of a single plateau in hydrogen desorption pressure-composition isotherms confirms single step hydrogen absorption-desorption behavior in Zr_0.8Ti_0.2-xNb_xCo alloys. The equilibrium pressure of hydrogen desorption decreases marginally with increasing Nb content in Zr_0.8Ti_0.2-xNb_xCo alloys which is further corroborated by differential scanning calorimetry measurements. Investigation of hydrogen induced disproportionation behavior in ITER-simulating condition revealed substantial impact of co-substitution of Ti and Nb on anti-disproportionation properties of ZrCo alloy. These remarkable properties make the Ti and Nb co-substituted quaternary alloys a desirable material for hydrogen isotope storage and delivery application.     

Hydrogenation and dehydrogenation behaviours of nanocrystalline Mg20Ni10−xCux (x = 0−4) alloys prepared by melt spinning

In order to improve the hydriding and dehydriding kinetics of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Cu, and the nanocrystalline Mg₂Ni-type Mg₂₀Ni_10−xCu_x (x = 0, 1, 2, 3, 4) alloys were synthesized by melt-spinning technique. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results show that the substitution of Cu for Ni does not change the major phase Mg₂Ni. The hydrogen absorption capacity of the alloys first increases and then decreases with rising Cu content, but the hydrogen desorption capacity of the alloys grows with increasing Cu content. The melt spinning significantly improves the hydrogenation and dehydrogenation capacity and kinetics of the alloys.